Electrolytic polishing of stainless steel



United States Patent 3,342,711 ELECTROLYTIC PgLlsHIlNG 0F STAINLESS TEEAkio Shiga and Hideyuki Furukawa, Machida-shi, Japan, assignors to KyowaHakko Kogyo Co., Ltd., Tokyo, Japan, a corporation of Japan No Drawing.Filed Nov. 13, 1964, Ser. No. 411,100 Claims priority, applicationJapan, Nov. 14, 1963, 38/ 60,874 4 Claims. (Cl. 204-1405) This inventionrelates to a method for electrolytically polishing metals and to asolution for electrolytic polishing of metals. More particularly thisinvention relates to a method for polishing metals electrolytically bythe use of a solution obtained by adding gluconic acid, alkali andalkali-earth metal (e.g. sodium and calcium) salts thereof, picoline orquinoline with or without ethylene glycol, to a mixture of phosphoricacid and sulfuric acid as an electrolyte solution, and constituting ametal to be polished as anode.

For the purpose of polishing metals electrolytically, there has beenused an electrolyte consisting mainly of phosphoric acid and sulfuricacid together with, as additive, chromic acid anhydride or glycerine. Insuch cases, e.g. where chromic acid anhydride is used, it is possible toobtain electrolytic products in lustrous form as long as the electrolytesolution is fresh, but with the prolonged continuation of electrolysis,the luster of the products begins to lose uniformity, apparently due tochange to chromic acid. When glycerine is used as additive, carrying outthe electrolysis operation at a high voltage and/or at a hightemperature causes a change of composition of the electrolyte solution,so that control of the composition of the electrolyte solution becomesdiflicult and products having uniform l'uster can no longer be obtained.

An object of the present'invention is, accordingly, to provide a methodfor polishing metals electrolytically, which method is free from theabove-mentioned drawbacks of the prior art methods. Another object ofthe present invention is to provide a method for yielding socalled 18,16 and 13 stainless steels having perfect lustre by electrolyticpolishing. This has been unsuccessful according to conventional methods.A further object of the present invention is to provide an electrolytesolution having no age deterioration and no change of composition, andwhich makes it possible to operate under a stabilized condition over along period of time. A still further object of the present invention isto provide a method for electrolytically polishing metals with a smallelectric current at a low voltage and at a low temperature. These andother objects can be realized by the present invention.

According to the present invention, a solution obtained byincorporating, as additive(s), at least one of gluconic acid, alkali andalkali-earth metal (eg sodium and calcium) salts thereof, picoline, orquinoline, with or without ethylene glycol, into a mixture of phosphoricacid and sulfuric acid, is used as an electrolyte solution and thepolishing of the metals is carried out while using the said metals asanodes.

The functions of the gluconic acid, salt thereof, picoline, saltthereof, and quinoline appears to be somewhat different from that of theethylene glycol in that the latter seems to act predominantly as astabilizer and secondarily as polishing activator, whereas the reverseis true for the other additives. It is therefore advantageous to haveethylene glycol present in all situations where a predominantstabilizing action is required. However, the results according to theinvention can be achieved with the use of ethylene glycol alone or withany one or more of the other additives in the absence of ethyleneglycol.

The phosphoric acid and sulfuric acid used in the present invention arepreferably of a concentration greater than 85 percent and 95 percent,respectively. The proportion of mixing is advantageously 2 parts ofphosphoric acid to 1 part of sulfuric acid (by volume) but aconsiderable variation is allowable, While still achieving the objectsof the invention. Into this acid mixture, various additives areincorporated, i.e. at least one additive selected from theabove-mentioned group is incorporated into the acid mixture. The amountof additive is variable in accordance with the composition thereof; seethe illustrative examples hereinafter disclosed. Accordingly it is alsoto be understood that the amounts of additives are not restricted tothose disclosed by way of illustration in the examples.

As regards metals to be polished, various kinds of metals can beobjectives of the present method. The present method affordsparticularly excellent effect upon various kinds of stainless steel,aluminum, aluminum alloys and the like.

In the practice of the present electrolytic polishing method, theelectrolysis is carried out by the use of the above-mentioned liquid asan electrolyte solution, with the article to be polished as anode, andlead, lead allay, carbon or the like as cathode. The voltage ispreferably more than 7 volts, and the amperage is preferably more than 5amperes/dm. There is no limit as to the elec trolysis temperature but atemperature about 40 C. is preferable. There is also no limit to theelectrolysis time but the polishing will be finished usually in severalminutes. These conditions are variable according to the composition ofthe electrolyte solution and the substance to be polished but thoseskilled in the art can readily select the most suitable conditions.

The following examples, wherein the procedure is as set forth in thepreceding paragraph, are given to illustrate the present inventionwithout limiting its scope:

EXAMPLES-COMPOSITION OF ELECTROLYTE SOLUTION FOR POLISHING PhosphoricGluconic Acid Acid Acid Sodium Gluconate (a) (a) Potassium CalciumGluconate Gluconate (a) (a) Ethylene Picoline Quinoline Glycol (g.) (g.)

NOTE 1.The abbreviations cc. and g. stand for cubic centimeters andgrams, respectively.

NOTE 2.The concentration of the sulfuric acid is 95%.

The concentration of the phosphoric acid is CONDITIONS OF ELECTROLYTICPOLISHING Example Anode Cathode Voltage Current Tempera- Time State ofpolished No. (v.) (A/dmfl) ture 0.) (min.) substance I 18-8 stainlesssteel Carbon 7 5 6 Mirror surface.

.do d 7 12 15 3 Do. 10 8 3 Do. 20 20 70 1 Do. 10 12 2 D0. 7 20 60 3 Do.7 20 40 3 Do. 18 or 16, 13 stainless steel 7 20 20 3 Do. 18-8 or 18, 16,16, stainless steel 7 30 60 3 Do. Aluminum L 7 20 3 Do. 18-8 or 18, 16,13, stainless steel 10 40 3 Do. 18-8 stainless steel L d 7 20 50 5 Do.do 7 30 5 Do. 7 30 7O 5 Do. 7 40 60 5 Do.

No'rE.v.=volts; A/dmfi =amperes per square decimeter; min.=minutes.

What is claimed is:

1. A method for polishing stainless steel which cornprises electrolyzinga solution consisting essentially of a minor proportion of (a) at leastone member selected from the group consisting of gluconic acid, alkaliand alkaline earth metal salts thereof and picoline, and (b) ethyleneglycol, and a predominant proportion of concentrated aqueous phosphoricacid and concentrated aqueous sulfuric acid, with the metal to bepolished constituting the anode.

2. A method for polishing stainless steel according to claim 1 in whichthe voltage is more than 7 volts, the amperage is more than 5amperes/dm. and the temperature is above 40 C.

3. A method for polishing stainless steel according to claim 1 in whichthe initial concentration of the phosphoric acid is more than by weightand the initial concentration of the sulfuric acid is more than byweight.

4. A method in accordance with claim 1 wherein the 20 proportion ofphosphoric acid to sulfuric acid is about 2:1

by volume, and the solution contains about 10 grams of sodium gluconateand about 50 grams of ethylene glycol for each 400 milliliters ofconcentrated sulfuric acid.

References Cited UNITED FOREIGN PATENTS Australia. Great Britain.

JOHN H. MACK, Primary Examiner.

40 R. K. MIHALEK, Assistant Examiner.

1. A METHOD FOR POLISHING STAINLESS STEEL WHICH COMPRISES ELECTROLYZINGA SOLUTION CONSISTING ESSENTIALLY OF A MINOR PROPORTION OF (A) AT LEASTONE MEMBER SELECTED FROM THE GROUP CONSISTING OF GLUCONIC ACID, ALKALIAND ALKALINE EARTH METAL SALTS THEREOF AND PICOLINE, AND (B) ETHYLENEGLYCOL, AND A PREDOMINANT PROPORTION OF CONCENTRATED AQUEOUS PHOSPHORICACID AND CONCENTRATED AQEUOUS SULFURIC ACID, WITH THE METAL TO BEPOLISHED CONSTITUTING THE ANODE.